A conventional mobile communication system of the LTE (Long Term Evolution) scheme specified by the 3GPP is configured to communicate between a mobile station UE and a radio base station eNB using a certain key.
The certain key includes, for example, a key KRRC—Ciph used for “Ciphering” in an RRC protocol, which is a C-plane protocol between the mobile station UE and the radio base station eNB (Access Stratum, AS), a key KRRC—IP used for “Integrity Protection” in the RRC protocol, and a key KUP—Ciph used for “Ciphering” in a U-plane protocol between the mobile station UE and the radio base station eNB (Access Stratum, AS) and the like. These certain keys are generated using a first key KeNB.
Using the same key as any of the certain keys and the first key KeNB for a long time is not preferable, because it makes the system's security vulnerable. For this reason, a procedure for updating such a certain key or a first key KeNB during a handover procedure or a re-establishment procedure is being reviewed by the 3GPP.
The handover procedures and the re-establishment procedures presently being reviewed by the applicant are shown in FIG. 7 to FIG. 9. Specifically, FIG. 7 shows an Inter-eNB handover procedure (X2 handover procedure, handover procedure between different radio base stations), FIG. 8 shows an Intra-eNB re-establishment procedure (intra-radio base station re-establishment procedure), and FIG. 9 shows an Inter-eNB re-establishment procedure (re-establishment procedure between different radio base stations).